The invention relates to a device and a method for increasing the angular resolution in a synthetic aperture radar system (SAR) looking in the direction of motion and laterally thereto.
The invention relates to an imaging radar system on board a moving platform. The object of the radar system is the imaging of a terrain lying in the direction of motion and of objects disposed thereon. The moving platform on which the radar system is mounted can be, for example, an aircraft, an unmanned missile, a satellite or a land vehicle. The radar system comprises a device for generating the transmitted signal, a transmitting antenna system, a receiving antenna system and a signal-processing device for evaluating the backscattered signals. The maximum size of the aperture of the transmitting antenna system and of the receiving antenna system is predetermined unchangeably by the space available in the moving platform. The resulting antenna characteristic with which the terrain to be imaged is illuminated is determined by superposition of the antenna characteristics of the transmitting antenna system and of the receiving antenna system.
Knowing the half-value width 2xcex13dB of the resulting antenna characteristic, the theoretical azimuth resolution xcex94RAR for such an imaging radar system is expressed as follows on the basis of the antenna characteristic:
xcex94xcfx86RARxcx9c2xcex13dBxe2x80x83xe2x80x83(1)
This resolution xcex94xcfx86RAR is also known as real aperture radar resolution (J. C. Curlander, R. N. McDonough; Synthetic Aperture Radar, John Wiley and Sons, Inc., New York, 1991). The real aperture radar resolution xcex94xcfx86RARmax is identical for all azimuth directions. The maximum real aperture radar resolution xcex94xcfx86RAR is limited by the maximum space available and is expressed by the maximum horizontal aperture width 2"xgr"max normalized to the transmission wavelength and thus the minimum attainable half-value width 2xcex1dBmin                               2          ⁢                      α                          3              ⁢              dBmin                                      =                              1                          2              ⁢                              ζ                max                                              ·                                    180              ⁢              xc2x0                        π                                              (        2        )                                          Δϕ          RARmax                =                              2            ⁢                          α                              3                ⁢                dBmin                                              =                                    1                              2                ⁢                                  ξ                  max                                                      ·                                          180                ⁢                xc2x0                            π                                                          (        3        )            
Since the real aperture radar resolution xcex94xcfx86RARmax is limited, resolution in the case of forward-looking radar systems also takes place on the basis of the doppler frequencies in azimuth direction. With the transmission frequency fc, the platform velocity v, the velocity of light c0, the half-value width 2xcex13dB and the angular velocity xcfx89 of the resulting antenna characteristic, the theoretically possible azimuth resolution is expressed on the basis of the doppler frequencies in the azimuth viewing direction xcfx86 (Wehner, R. D.: High-resolution radar, Artech House, Norwood, 1995):                                           Δϕ            DBS                    |                                    -                                                                    c                    0                                    ·                  ω                                                  4                  ⁢                                                            α                                              3                        ⁢                        db                                                              ·                    v                    ·                                          f                      c                                        ·                    si                                    ⁢                                      xe2x80x83                                    ⁢                                      μ                    ⁡                                          (                      ϕ                      )                                                                                            ·                                                                                180                    ⁢                    xc2x0                                                                                                π                                                                    ,                            (        4        )            
This resolution xcex94xcfx86DBS is also known as the doppler beam sharpening resolution. In FIG. 1 there are illustrated, as a function of azimuth viewing direction xcfx86, the resolutions xcex94xcfx86RAR according to (1) and xcex94xcfx86DBS according to (4) for the values fc=10 GHz, v=100 m/s, xcfx89=40xc2x0/s and 2xcex13dB32 1xc2x0 and 3xc2x0 respectively used as examples. It is shown that, with decreasing half-value width 2xcex13dB, the real aperture radar resolution xcex94xcfx86RAR becomes better, although at the same time the doppler beam sharpening resolution xcex94xcfx86DBS deteriorates. From FIG. 1 it can also be inferred that, for large xcfx86, the doppler beam sharpening resolution xcex94xcfx86DBS is higher than the real aperture radar resolution xcex94xcfx86RAR. The angular region in which the doppler beam sharpening resolution xcex94xcfx86DBS is poorer than the real aperture radar resolution xcex94xcfx86RAR is known as the xe2x80x9cblind sectorxe2x80x9d.
When the SAR is observing straight outward, directly in flying direction, objects that lie within the half-value width 2xcex13dB of the antenna are no longer clearly located, since objects lying both right and left of the flying direction have identical doppler frequency shifts. This leads to an xe2x80x9cazimuth ambiguityxe2x80x9d (J. C. Curlander, R. N. McDonough, Synthetic Aperture Radar, John Wiley and Sons, Inc., New York, 1991).
Conventional radar systems must therefore find a compromise between good real aperture resolution to minimize the xe2x80x9cazimuth ambiguityxe2x80x9d and a small xe2x80x9cblind sectorxe2x80x9d. For this purpose it must be ensured that the angular region of xe2x80x9cazimuth ambiguityxe2x80x9d does not overlap with the angular region within which resolution must be achieved on the basis of doppler frequencies. In practice, this means that the radar system must be designed such that the angular region of xe2x80x9cazimuth ambiguityxe2x80x9d is not greater than the xe2x80x9cblind sectorxe2x80x9d. Furthermore, conventional radar systems must have a logic system that informs the signal-evaluating system of the angular direction at which resolution with real aperture resolution can become better than with doppler beam sharpening in azimuth direction and vice versa.
The object of the invention is to find a method as well as a suitable device for performing the method which reduces both the xe2x80x9cblind sectorxe2x80x9d and the region of xe2x80x9cazimuth ambiguityxe2x80x9d. Another object of the invention is to circumvent the need for a logic system that informs the signal-evaluating system of the azimuth viewing direction xcfx86 at which resolution with real aperture resolution becomes better than with doppler beam sharpening in azimuth direction.
In the inventive method for increasing the angular resolution in a radar system which evaluates the doppler frequency shift of the transmitted signal in order to increase the angular resolution, a plurality of adjacent, narrow regions are sampled sequentially by means of a pencil-beam antenna characteristic during movement of the radar system. Such sampling takes place so rapidly that it is equivalent to illumination of the total area of all regions by means of an antenna with broad-beam antenna characteristic. Thus the antenna characteristic of this simulated, broad-beam illumination corresponds to superposition of the individual antenna characteristics which sample narrowly bounded regions. The method evaluates both the signals resulting from the pencil-beam sampling process and the simulated broad-beam antenna signal resulting by means of superposition of the individual sequential, pencil-beam sampling processes. On the basis of the instantaneous viewing angle of the radar system, a decision is advantageously made as to whether the signals resulting from the pencil-beam sampling process will be evaluated individually, or whether the simulated broadband antenna signal will be used. The large angular region, which is illuminated almost simultaneously, leads to an illumination time that is adequate for calculation of the doppler frequency shift. At the same time, good real aperture resolution can also be achieved on the basis of the evaluation of the pencil-beam antenna characteristics.